Integrated pathophysiology of schizophrenia, major depression, and bipolar disorder as monoamine axon disorder

Recent studies provide evidence that similar to early-stage Parkinson's disease, depression is a neurodegenerative disease characterized by the degeneration of monoamine axons. The major difference between the two disorders is that the symptoms of depression become evident without loss of monoamine neurons, while the motor symptoms of Parkinson's disease appear after loss of the cell body. Given that the axonal degeneration of monoamine neurons underlies the pathophysiology of neurological (Parkinson's disease) and neuropsychiatric (depression) diseases, axonal impairment of monoamine neurons is thought to also occur in schizophrenia and bipolar disorder and play a significant role in the pathophysiology of these mental illnesses. The positive symptoms of schizophrenia and manic symptoms of bipolar disorder are known to occur in hyper-monoaminergic states, opposite to depressive symptoms, negative/cognitive symptoms of schizophrenia, and motor disorders of Parkinson's disease, all occurring in hypo-monoaminergic states. Since monoamine axons have the capacity to spontaneously regenerate or sprout in response to damage in the adult brain and sometimes show hyperinnervation due to excessive regeneration/sprouting beyond normal levels, it is possible that schizophrenia and bipolar disorder are disorders that include excessive regeneration/sprouting of monoamine axons leading to hyper-monoaminergic states. Together, based on accumulating data from animal and human studies, the pathophysiology of schizophrenia, major depression, and bipolar disorder is summarized as follows: The degeneration of monoamine axons is associated with the negative and cognitive symptoms of schizophrenia, major and bipolar depression, while hyper-regeneration/sprouting of monoamine axons underlies the positive symptoms of schizophrenia and bipolar mania. The integrated understanding of schizophrenia, major depression, and bipolar disorder as monoamine axon disorder will open the door to the development of new diagnosis and treatment methods for major mental illnesses as well as early-stage Parkinson's disease.